Superelastic guiding member

ABSTRACT

An improved guiding member for use within a body lumen having a unique combination of superelastic characteristics. The superelastic alloy material has a composition consisting of about 30% to about 52% (atomic) titanium, and about 38% to 52% nickel and may have one or more elements selected from the group consisting of iron, cobalt, platinum, palladium, vanadium, copper, zirconium, hafnium and niobium. The alloy material is subjected to thermomechanical processing which includes a final cold working of about 10 to about 75% and then a heat treatment at a temperature between about 450° and about 600° C. and preferably about 475° to about 550° C. Before the heat treatment the cold worked alloy material is preferably subjected to mechanical straightening. The alloy material is preferably subjected to stresses equal to about 5 to about 50% of the room temperature ultimate yield stress of the material during the thermal treatment. The guiding member using such improved material exhibits a stress-induced austenite-to-martensite phase transformation at an exceptionally high constant yield strength of over 90 ksi for solid members and over 70 ksi for tubular members with a broad recoverable strain of at least about 4% during the phase transformation. An essentially whip free product is obtained.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part application toapplication Serial No. 07/629,381, filed Dec. 18, 1990, entitledSUPERELASTIC GUIDING MEMBER.

BACKGROUND OF THE INVENTION

[0002] This invention relates to the field of medical devices, and moreparticularly to guiding means such as guidewires for advancing catheterswithin body lumens in procedures such as percutaneous transluminalcoronary angioplasty (PTCA).

[0003] In typical PTCA procedures a guiding catheter having a preformeddistal tip is percutaneously introduced into the cardiovascular systemof a patient in a conventional Seldiger technique and advanced thereinuntil the distal tip of the guiding catheter is seated in the ostium ofa desired coronary artery. A guidewire is positioned within an innerlumen of a dilatation catheter and then both are advanced through theguiding catheter to the distal end thereof. The guidewire is firstadvanced out of the distal end of the guiding catheter into thepatient's coronary vasculature until the distal end of the guidewirecrosses a lesion to be dilated, then the dilatation catheter having aninflatable balloon on the distal portion thereof is advanced into thepatient's coronary anatomy over the previously introduced guidewireuntil the balloon of the dilatation catheter is properly positionedacross the lesion. Once in position across the lesion, the balloon isinflated to a predetermined size with radiopaque liquid at relativelyhigh pressures (e.g. greater than 4 atmospheres) to compress thearteriosclerotic plaque of the lesion against the inside of the arterywall and to otherwise expand the inner lumen of the artery. The balloonis then deflated so that blood flow is resumed through the dilatedartery and the dilatation catheter can be removed therefrom.

[0004] Conventional guidewires for angioplasty and other vascularprocedures usually comprise an elongated core member with one or moretapered sections near the distal end thereof and a flexible body such asa helical coil disposed about the distal portion of the core member. Ashapable member, which may be the distal extremity of the core member ora separate shaping ribbon which is secured to the distal extremity ofthe core member extends through the flexible body and is secured to arounded plug at the distal end of the flexible body. Torquing means areprovided on the proximal end of the core member to rotate, and therebysteer, the guidewire while it is being advanced through a patient'svascular system.

[0005] Further details of dilatation catheters, guidewires, and devicesassociated therewith for angioplasty procedures can be found in U.S.Pat. No. 4,323,071 (Simpson etal.); U.S. Pat. No. 4,439,185 (Lundquist);U.S. Pat. No. 4,516,972 (Samson); U.S. Pat. No. 4,538,622 (Samson etal.); U.S. Pat. No. 4,554,929 (Samson et al.); U.S. Pat. No. 4,616,652(Simpson); and U.S. Pat. No. 4,638,805 (Powell).

[0006] Steerable dilatation catheters with fixed, built-in guidingmembers, such as described in U.S. Pat. No. 4,582,181 (now Re 33,166)are frequently used because they have lower deflated profiles thanconventional over-the-wire dilatation catheters and a lower profileallows the catheter to cross tighter lesions and to be advanced muchdeeper into a patient's coronary anatomy.

[0007] A major requirement for guidewires and other guiding members,whether they be solid wire or tubular members, is that they havesufficient columnar strength to be pushed through a patient's vascularsystem or other body lumen without kinking. However, they must also beflexible enough to avoid damaging the blood vessel or other body lumenthrough which they are advanced. Efforts have been made to improve boththe strength and flexibility of guidewires to make them more suitablefor their intended uses, but these two properties are for the most partdiametrically opposed to one another in that an increase in one usuallyinvolves a decrease in the other.

[0008] The prior art makes reference to the use of alloys such asNITINOL, which is an acronym for Ni-Ti Naval Ordnance Laboratory. Thesealloys have shape memory and/or superelastic characteristics and may beused in medical devices which are designed to be inserted into apatient's body. The shape memory characteristics allow the devices to bedeformed to facilitate their insertion into a body lumen or cavity andthen be heated within the body so that the device returns to itsoriginal shape. Superelastic characteristics on the other hand generallyallow the metal to be deformed and restrained in the deformed conditionto facilitate the insertion of the medical device containing the metalinto a patient's body, with such deformation causing the phasetransformation. Once within the body lumen the restraint on thesuperelastic member can be removed, thereby reducing the stress thereinso that the superelastic member can return to its original undeformedshape by the transformation back to the original phase.

[0009] Alloys having shape memory/superelastic characteristics generallyhave at least two phases. These phases are a martensite phase, which hasa relatively low tensile strength and which is stable at relatively lowtemperatures, and an austenite phase, which has a relatively hightensile strength and which is stable at temperatures higher than themartensite phase.

[0010] Shape memory characteristics are imparted to the alloy by heatingthe metal at a temperature above which the transformation from themartensite phase to the austenite phase is complete, i.e. a temperatureabove which the austenite phase is stable. The shape of the metal duringthis heat treatment is the shape “remembered”. The heat treated metal iscooled to a temperature at which the martensite phase is stable, causingthe austenite phase to transform to the martensite phase. The metal inthe martensite phase is then plastically deformed, e.g. to facilitatethe entry thereof into a patient's body. Subsequent heating of thedeformed martensite phase to a temperature above the martensite toaustenite transformation temperature causes the deformed martensitephase to transform to the austenite phase and during this phasetransformation the metal reverts back to its original shape.

[0011] The prior methods of using the shape memory characteristics ofthese alloys in medical devices intended to be placed within a patient'sbody presented operational difficulties. For example, with shape memoryalloys having a stable martensite temperature below body temperature, itwas frequently difficult to maintain the temperature of the medicaldevice containing such an alloy sufficiently below body temperature toprevent the transformation of the martensite phase to the austenitephase when the device was being inserted into a patient's body. Withintravascular devices formed of shape memory alloys havingmartensite-to-austenite transformation temperatures well above bodytemperature, the devices could be introduced into a patient's body withlittle or no problem, but they had to be heated to themartensite-to-austenite transformation temperature which was frequentlyhigh enough to cause tissue damage and very high levels of pain.

[0012] When stress is applied to a specimen of a metal such as NITINOLexhibiting superelastic characteristics at a temperature above which theaustenite is stable (Le. the temperature at which the transformation ofmartensite phase to the austenite phase is complete), the specimendeforms elastically until it reaches a particular stress level where thealloy then undergoes a stress-induced phase transformation from theaustenite phase to the martensite phase. As the phase transformationproceeds, the alloy undergoes significant increases in strain but withlittle or no corresponding increases in stress. The strain increaseswhile the stress remains essentially constant until the transformationof the austenite phase to the martensite phase is complete. Thereafter,further increase in stress are necessary to cause further deformation.The martensitic metal first yields elastically upon the application ofadditional stress and then plastically with permanent residualdeformation.

[0013] If the load on the specimen is removed before any permanentdeformation has occurred, the martensitic specimen will elasticallyrecover and transform back to the austenite phase. The reduction instress first causes a decrease in strain. As stress reduction reachesthe level at which the martensite phase transforms back into theaustenite phase, the stress level in the specimen will remainessentially constant (but substantially less than the constant stresslevel at which the austenite transforms to the martensite) until thetransformation back to the austenite phase is complete, i.e. there issignificant recovery in strain with only negligible corresponding stressreduction. After the transformation back to austenite is complete,further stress reduction results in elastic strain reduction. Thisability to incur significant strain at relatively constant stress uponthe application of a load and to recover from the deformation upon theremoval of the load is commonly referred to as superelasticity orpseudoelasticity.

[0014] The prior art makes reference to the use of metal alloys havingsuperelastic characteristics in medical devices which are intended to beinserted or otherwise used within a patient's body. See for example,U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamotoet al.).

[0015] The Sakamoto et al. patent discloses the use of a nickel-titaniumsuperelastic alloy in an intravascular guidewire which could beprocessed to develop relatively high yield strength levels. However, atthe relatively high yield stress levels which cause theaustenite-to-martensite phase transformation characteristic of thematerial, it did not have a very extensive stress-induced strain rangein which the austenite transforms to martensite at relative constantstress. As a result, frequently as the guidewire was being advancedthrough a patient's tortuous vascular system, it would be stressedbeyond the superelastic region, i.e. develop a permanent set or evenkink which can result in tissue damage. This permanent deformation wouldgenerally require the removal of the guidewire and the replacementthereof with another.

[0016] Products of the Jervis patent on the other hand had extensivestrain ranges, i.e. 2 to 8% strain, but the relatively constant stresslevel at which the austenite transformed to martensite was very low,e.g. 50 ksi.

[0017] What has been needed and heretofore unavailable is an elongatedsolid or tubular body for intravascular devices, such as guide wires orguiding members, which have at least a portion thereof exhibitingsuperelastic characteristics including an extended strain region over arelatively constant high stress level which effects the austenitetransformation to martensite and still provide a one-to-one torqueresponse. The present invention satisfies these and other needs.

SUMMARY OF THE INVENTION

[0018] The present invention is directed to an improved superelasticbody which is suitable for intraluminal devices, such as guidewires orguiding members, wherein superelastic characteristics result from thestress-induced transformation of austenite to martensite.

[0019] The superelastic alloy body of the invention has a stableaustenite phase which will transform to martensite phase upon theapplication of stress and will exhibit a recoverable strain of at leastabout 4% upon the stress induced transformation of the austenite phaseto the martensite phase. The formation of the alloy body includes afinal cold working about 10 to about 75% and then a final memoryimparting heat treatment at a temperature of about 450° to about 600°C., preferably 475° to about 550° C. The cold worked, heat treatedproduct exhibits a stress-induced phase transformation at temperaturesbelow about 45° C. at a relatively high stress level, e.g. above about70 ksi, (483 Mpa) preferably above about 90 ksi (620 Mpa) for solidproducts and about 40 ksi (276 Mpa) for hollow tubular products. Thesuperelastic product exhibits at recoverable strain of at least 4% uponcompletion of the stress-induced transformation of the austenite phaseto the martensite phase. The onset of the stress induced phase changefrom austenite to martensite, preferably begins when the specimen hasbeen strained about 2% and extends to a strain level of about 8% at thecompletion of the phase change. The stress and strain referred to hereinis measured by tensile testing. The stress-strain relationshipdetermined by applying a bending moment to a cantilevered specimen isslightly different from the relationship determined by tensile testingbecause the stresses which occur in the specimen during bending are notas uniform as they are in tensile testing. The stress change during thephase transformation is much less than the stress either before or afterthe stress-induced transformation. In some instances the stress levelduring the phase change is almost constant.

[0020] The elongated portion of the guiding member having superelasticproperties is preferably formed from an alloy consisting essentially ofabout 30 to about 52% titanium, about 38% to about 52% nickel andadditional alloying elements in amount up to 20% for copper and up toabout 10% in the case of other alloying elements. The additionalalloying elements may be selected from the group consisting of up to 3%each of iron, cobalt, chromium, platinum, palladium, zirconium, hafniumand niobium and up to about 10% vanadium. At nickel levels above 52% thealloy becomes too brittle to fabricate by cold working. Metallurgicallythe alloy consist essentially of a predominant quantity of a NiTiintermetallic compound and small quantities of other constituents.Additionally, when nickel is in excess Ni₃Ti is formed, whereas whentitanium is in excess Ti₂Ni is formed. As used herein, all references topercent alloy compositions are atomic percent unless otherwise noted.

[0021] To form the elongated superelastic portion of the guiding member,elongated solid rod or tubular stock of the preferred alloy material isfirst thermomechanically processed through a series of cold working,e.g. drawing and inter-annealing at temperatures between about 600° toabout 800° C. for about 5 to about 30 minutes and is then given a finalcold working, e.g. drawing, to effect a final size reduction of about10% up to about 75% in the transverse cross section thereof. For solidproducts the final cold work is preferably about 30 to about 70% and forhollow tubular products the final cold work is preferably about 10% toabout 40%. As used herein % cold work is the size reduction of thetransverse dimension of the work piece effected by the cold working.After the final cold working, the material is given a heat treatment ata temperature of about 450° to about 600° C., preferably about 475° toabout 550° C., for about 0.5 to about 60 minutes to generate thesuperelastic properties. To impart a straight memory, the cold workedmaterial may be subjected to a longitudinal stress equal to about 5% toabout 50%, preferably about 10% to about 30%, of the yield stress of thematerial (as measured at room temperature) during a heat treatment ofabout 450° to about 600° C. This thermomechanical processing imparts astraight “memory” to the superelastic portion and provides a relativelyuniform residual stress in the material. Preferably, the final coldworked product is subjected to mechanically straightening between thefinal cold working and heat treating steps to provide the wire ortubular product with substantially improved, one-to-one torque response,i.e. it is substantially whip free. The alloy composition and thermaltreatment are selected to provide an austenite finish transformationtemperature generally about −20° to about 40° C. and usually less thanbody temperature (approx. 37° C.). To obtain more consistent finalproperties, it is preferred to fully anneal the solid rod or tubularstock prior to cold working so that the material will always have thesame metallurgical structure at the start of the cold working. Thepre-annealing also ensures adequate ductility for cold working. It willbe appreciated by those skilled in the art that the alloy can be coldworked in a variety of ways other than drawing, such as rolling orswaging. The constant stress levels for tubular products have been foundto be slightly lower than the constant stress levels for solid productsdue to the inability to cold work the tubular products to the extent thesolid products can be cold worked. For example, solid superelastic wirematerial of the invention can have a relatively constant stress levelabove about 70 ksi, usually above about 90 ksi, whereas, hollowsuperelastic tubing material of the invention can have a relativelyconstant stress level above about 50 ksi (345 Mpa) usually above about60 ksi (414 Mpa). The ultimate tensile strength of both forms of thematerial is well above 200 ksi (1380 Mpa) with an ultimate elongation atfailure of about 15%.

[0022] The elongated body of the invention exhibits a stress-inducedaustenite-to-martensite phase transformation over a broad region ofstrain at very high, relatively constant stress levels. As a result aguiding member formed of this material is very flexible, it can beadvanced through very tortuous passageways such as a patient's coronaryvasculature with little risk that the superelastic portion of theguiding member will develop a permanent set and at the same time it willeffectively transmit the torque applied thereto without causing theguiding member to whip.

[0023] These and other advantages of the invention will become moreapparent from the following detailed description thereof when taken inconjunction with the following exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 illustrates a guidewire which embodies features of theinvention.

[0025]FIG. 2 illustrates another embodiment of a guidewire of theinvention.

[0026]FIG. 3 is a partial side elevational view, partially in section,of a guiding member embodying features of the invention which isincorporated into a fixed-wire dilatation catheter adapted for balloonangioplasty procedures.

[0027]FIG. 4 is a schematic, graphical illustration of the stress-strainrelationship of superelastic material.

[0028]FIG. 5 illustrates another embodiment of a guidewire embodyingfeatures of the invention.

[0029]FIG. 6 illustrates the effects of the temperature during heattreatment after final cold working on the final austenite transformationtemperature (Af).

DETAILED DESCRIPTION OF THE INVENTION

[0030]FIG. 1 illustrates a guidewire 10 embodying features of theinvention that is adapted to be inserted into a body lumen such as anartery. The guidewire 10 comprises an elongated body or core member 11having an elongated proximal portion 12 and a distal portion 13, atleast part of which, preferably the distal portion, is formed ofsuperelastic material of the invention. The distal portion 13 has aplurality of sections 14, 15 and 16 having sequentially smallerdiameters with tapered sections 17, 18 and 19 connecting the smallerdiameter sections with adjacent sections. The elongated proximal portion12 is provided with a female distal end 20 which receives the male end21 of the distal portion 13. The ends 20 and 21 may be press fittogether or may be secured together by means such as a suitable adhesiveor by welding, brazing or soldering.

[0031] A helical coil 22 is disposed about the distal portion 13 and hasa rounded plug 23 on the distal end thereof where the distal end of thehelical coil is welded to the distal end of a shaping ribbon 24 which issecured by its proximal end to the distal portion 13 by suitable means(e.g. brazing) at location 25. The coil 22 is also secured to the distalportion 13 of the elongated body 11 at location 25 and to the taperedsection 17 at location 26. Preferably, the most distal section 27 of thehelical coil 22 is made of radiopaque metal such as platinum or alloysthereof to facilitate the fluoroscopic observation of the distal portionof the guidewire while it is disposed within a patient's body.

[0032] The exposed portion of the elongated body 11 should be providedwith a coating 28 of lubricous material such as polytetrafluoroethylene(sold under the trademark Teflon by du Pont) or other suitable lubricouscoatings such as the polysiloxane coatings disclosed in copendingapplication Ser. No. 559,373, filed Jul. 24, 1990 which is herebyincorporated by reference.

[0033]FIG. 2 illustrates another embodiment of a guidewire whichincorporates features of the invention. This embodiment is very similarto the embodiment shown in FIG. 1 except that the entire elongated body11 is formed of material having superelastic characteristics and thedistal portion 13 of the core member 11 extends all the way to the plug23 and is preferably flattened at its most distal extremity 29 as ribbon24 in the embodiment shown in FIG. 1. All of the parts of the guidewireshown in FIG. 2 which correspond to the parts shown in FIG. 1 arenumbered the same as in FIG. 1.

[0034]FIG. 3 illustrates a fixed wire, steerable dilatation catheter 40which has incorporated therein a guiding member 41 in accordance withthe invention. In this embodiment, the catheter 40 includes an elongatedtubular member 42 having an inner lumen 43 extending therein and aninflatable, relatively inelastic dilatation balloon 44 on the distalextremity of the tubular member. Guiding member 41 which includes anelongated body 45, a helical coil 46 disposed about and secured to thedistal end of the elongated body 45 and a shaping ribbon 50 extendingfrom the distal end of the elongated body to rounded plug 51 at thedistal end of the coil 46. The proximal portion 52 of the elongated body45 is disposed within the inner lumen 43 of the tubular member 42 andthe distal portion 53 of the elongated body 45 extends through theinterior of the dilatation balloon 44 and out the distal end thereof.The distal end of the balloon 44 is twisted and sealed about the distalportion 53 of the elongated body 45 extending therethrough in a mannerwhich is described in more detail in copending application Ser. No.521,103, filed May 9, 1990, which is hereby incorporated by reference.The helical coil 46 is secured to the distal portion 53 of the elongatedbody 45 by suitable means such as brazing at location 54 which is thesame location at which the shaping ribbon 50 is secured to the distalportion of the elongated body. Preferably, the distal portion 53 is freeto rotate within the twisted seal of the distal end of the balloon 44and means are provided to seal the distal portion 53 therein to allowair to be vented therethrough but not inflation fluid such as shown inU.S. Pat. No. 4,793,350 (Mar et al.). The proximal end of the catheter40 is provided with a multiple arm adapter 55 which has one arm 56 fordirecting inflation fluid through the inner lumen 43 and the interior ofthe balloon 44. The proximal end of the elongated body 45 extendsthrough the adapter 55 and is secured to the torquing handle 57 whichrotates the guiding member 41 within the catheter 40 as the catheter isadvanced through a patient's vascular system. The tubular member 42 maybe formed of suitable plastic material such as polyethylene or polyamideor metals such as stainless steel or NITINOL. All or at least the distalportion of the tubular member 42 may be formed of the superelastic NiTitype alloy material of the invention. In an alternative construction,the elongated body 45 has a flattened distal portion which is secured tothe rounded plug 51.

[0035] The elongated body 11 of the guidewire 10 and elongated body 45of the fixed-wire catheter 40 are generally about 150 to about 200 cm(typically about 175 cm) in length with an outer diameter of about 0.01to 0.018 inch (0.25-0.46 mm) for coronary use. Larger diameter guidewireand guiding members may be employed in peripheral arteries. The lengthsof the smaller diameter sections 14, 15 and 16 can range from about 5 toabout 30 cm. The tapered sections 17, 18 and 19 generally are about 3 cmin length, but these too can have various lengths depending upon thestiffness or flexibility desired in the final product. The helical coils22 and 46 are about 20 to about 40 cm in length, have an outer diameterabout the same size as the diameter of the elongated bodies 11 and 45,and are made from wire about 0.002 to 0.003 inch (0.051-0.076 mm) indiameter. The last or most distal 1.5 to about 4 cm of the coil islongitudinally expanded and preferably made of platinum or otherradiopaque material to facilitate the fluoroscopic observation thereofwhen the guidewire or fixed wire catheter is inserted into a patient.The remaining portion of the coils 22 and 45 may be stainless steel. Thetransverse cross-section of the elongated bodies 11 and 45 is generallycircular. However, the shaping ribbons 24 and 50 and the flatteneddistal section 29 have rectangular transverse cross-sections whichusually have dimensions of about 0.001 by 0.003 inch (0.025-0.076 mm).

[0036] The superelastic guiding member of the invention, whether it isthe entire elongated body 11 or 45 or just a portion thereof, ispreferably made of an alloy material consisting essentially of about 30to about 52% titanium, about 38 to 50% nickel and the balance one ormore additional alloying elements in amounts of up to about 20%,preferably up to about 12% in the case of copper and up to 10% for otheradditional alloying elements. The other additional alloying elements maybe selected from the group consisting of iron, cobalt, platinum,palladium, zirconium, hafnium and niobium in amounts up to 3% each andvanadium in an amount of up to 10%. The addition of nickel aboveequiatomic amounts with titanium and the other identified alloyingelements increases the stress levels at which the stress-inducedaustenite-to-martensite transformation occurs and ensures that thetemperature at which the martensite phase transforms to the austenitephase is well below human body temperature so that austenite is the onlystable phase at body temperature. The excess nickel and additionalalloying elements also help to provide an expanded strain range at veryhigh stresses when the stress induced transformation of the austenitephase to the martensite phase occurs.

[0037] A presently preferred method for making the final configurationof the superelastic portion of the guiding member is to cold work,preferably by drawing, a rod or tubular member having a compositionaccording to the relative proportions described above to effect a finalsize reduction of about 10 to about 75% and then providing a memoryimparting heat treatment to the cold worked product at a temperature ofabout 450° to about 600° C. preferably about 475° to about 550° C., forabout 0.5 to about 60 minutes. In one preferred embodiment the coldworked product is subjected to tensile stress to hold the productstraight during the heat treatment to impart a straight memory thereto.In the embodiment with a solid wire product, the final cold workpreferably ranges from about 30 to about 70% and the heat treatmenttemperature ranges from about 450° to about 600° C., preferably about475° to about 550° C. In another presently preferred embodiment withtubular products, the final cold work ranges from about 10 to about 40%and the final memory imparting heat treatment temperature ranges are thesame as mentioned above. Typical initial transverse dimensions of therod or the tubular member prior to cold working are about 0.045 inch(1.14 mm) and about 0.25 inch (6.35 mm) respectively. If the finalproduct is to be a tubular product, a small diameter ingot, e.g. 0.20 toabout 1.5 inch (5.1-38.1 mm) in diameter and 5 to about 48 inches(12.7-122 cm) in length, may be formed into a hollow tube by extrudingor by machining a longitudinal center hole through a solid rod andgrinding the outer surface thereof smooth.

[0038] After each drawing step, except the last, the solid rod ortubular member is preferably annealed at a temperature of about 600° toabout 800° C., typically about 675° C., for about 15 minutes in air or aprotective atmosphere such as argon to relieve essentially all internalstresses. In this manner all of the specimens start the subsequentthermomechanical processing in essentially the same metallurgicalcondition so that consistent final properties are obtained. Suchtreatment also provides the requisite ductility for effective subsequentcold working. The stress relieved stock is preferably drawn through oneor more dies of appropriate inner diameter with a reduction per pass ofabout 10 to 70%. The final heat treatment fixes the austenite-martensitetransformation temperature. Mechanical straightening prior to the finalheat treatment, particularly when tension is applied during the lastheat treatment ensures a uniform level of residual stresses throughoutthe length of the superelastic material which minimizes or eliminatesguidewire whipping when made of this material when torqued within apatient's blood vessel.

[0039] In a typical procedure, starting with solid rod stock 0.100 inch(2.54 mm) in diameter, the cold working/heat treatment schedule would beperformed as follows:

[0040] 1. Cold work 60% from 0.100 inch to 0.0632 inch (1.61 mm) andanneal at 675° C. for 15 minutes.

[0041] 2. Cold work 60% form 0.0632 inch to 0.0399 inch (1.02 mm) andanneal at 675° C. for 15 minutes.

[0042] 3. Cold work 60% by drawing through 2-3 dies from 0.0399 inch to0.0252 inch (0.64 mm) and anneal at 675° C. for 15 minutes.

[0043] 5. Cold work 69% by drawing through 2-3 dies from 0.0252 inch to0.014 inch (0.36 mm).

[0044] 6. Mechanically straighten and then continuously heat treat at510° C. at 1 ft/min (0.0305 m/min) under sufficient tension to impart astraight memory.

[0045] When the cold worked material is subjected to the slightly lowerthermal treatments, it has substantially higher tensile properties andit exhibits stress-induced austenite to martensite phase transformationat very high levels of stress but the stress during the phasetransformation is not very constant. The addition of mechanicalstraightening prior to the final memory imparting heat treatment undertension will substantially improve the whipping characteristics of thefinal product.

[0046]FIG. 4 illustrates an idealized stress-strain relationship of analloy specimen having superelastic properties as would be exhibited upontensile testing of the specimen. The line from point A to point Bthereon represents the elastic deformation of the specimen. After pointB the strain or deformation is no longer proportional to the appliedstress and it is in the region between point B and point C that thestress-induced transformation of the austenite phase to the martensitephase begins to occur. There can be an intermediate phase developed,sometimes called the rhombohedral phase, depending upon the compositionof the alloy. At point C the material enters a region of relativelyconstant stress with significant deformation or strain. It is in theregion of point C to point D that the transformation from austenite tomartensite occurs. At point D the stress induced transformation to themartensite phase is substantially complete. Beyond point D thestress-induced martensite phase begins to deform, elastically at first,but, beyond point E, the deformation is plastic or permanent. If plasticdeformation occurs, the strain will not return to zero upon the removalof the stress.

[0047] When the stress applied to the superelastic metal is removed, themetal will recover to its original shape, provided that there was nopermanent deformation to the martensite phase. At point F in therecovery process, the metal begins to transform from the stress-induced,unstable martensite phase back to the more stable austenite phase. Inthe region from point G to point H, which is also an essentiallyconstant stress region, the phase transformation from martensite back toaustenite is essentially complete. The line from point I to the startingpoint A represents the elastic recovery of the metal to its originalshape.

[0048]FIG. 5 illustrates a guidewire 60 embodying features of theinvention which comprises an elongated, relatively high strengthproximal portion 61, a relatively short distal portion 62 which isformed substantially of superelastic alloy material and a tubularconnector element 63 which is formed substantially of superelastic alloymaterial and which connects the proximal end of the distal portion 62 tothe distal end of the proximal portion 61 into a torque transmittingrelationship. The distal portion 62 has at least one tapered section 64which becomes smaller in the distal direction. The connector element 63is a hollow tubular shaped element having an inner lumen extendingtherein which is adapted to receive the proximal end 65 of the distalportion 62 and the distal end 66 of the proximal portion 61. The ends 65and 66 may be press fit into the connector element 63 or they may besecured therein by crimping or swaging the connector element or by meanssuch as a suitable adhesive or by welding, brazing or soldering.

[0049] A helical coil 67 is disposed about the distal portion 62 and hasa rounded plug 68 on the distal end thereof. The coil 67 is secured tothe distal portion 62 at proximal location 70 and at intermediatelocation 71 by a suitable solder. A shaping ribbon 72 is secured by itsproximal end to the distal portion 62 at the same location 71 by thesolder and by the distal end thereof to the rounded plug 68 which isusually formed by soldering or welding the distal end of the coil 67 tothe distal tip of the shaping ribbon 72. Preferably, the distal section74 of the helical coil 67 is made of radiopaque metal such as platinumor platinum-nickel alloys to facilitate the observation thereof while itis disposed within a patient's body. The distal section 74 should bestretched about 10 to about 30%.

[0050] The most distal part 75 of the distal portion 62 is flattenedinto a rectangular section and preferably provided with a rounded tip76, e.g. solder, to prevent the passage of the most distal part throughthe spacing between the stretched distal section 74 of the helical coil67.

[0051] The exposed portion of the elongated proximal portion 61 shouldbe provided with a coating 77 of lubricous material such aspolytetrafluoroethylene (sold under the trademark Teflon by du Pont, deNemours & Co.) or other suitable lubricous coatings such as thepolysiloxane coatings disclosed in co-pending application Ser. No.559,373, filed Jul. 24, 1990 which is hereby incorporated by reference.

[0052] The elongated proximal portions of the guidewires of theinvention for coronary use are generally about 130 to about 140 cm inlength with outer diameters of about 0.006 to 0.018 inch (0.152-0.457mm). Larger diameter guidewires may be employed in peripheral arteriesand other body lumens. The lengths of the smaller diameter and taperedsections can range from about 2 to about 20 cm, depending upon thestiffness or flexibility desired in the final product. The helical coilsare about 20 to about 45 cm in length, have an outer diameter about thesame size as the diameter of the elongated proximal portions, and aremade from wire about 0.002 to 0.003 inch in diameter. The shapingribbons and the flattened distal sections of the distal portions haverectangular transverse cross-sections which usually have dimensions ofabout 0.001 by 0.003 inch.

[0053] An important feature of the present invention is the capabilityof maintaining the level of the yield stress which effects the stressinduced austenite-to-martensite transformation as high as possible, e.g.above 70 ksi, preferably above 90 ksi for solid products (lower withtubular products), with an extensive region of recoverable strain, e.g.a strain range of at least 4%, preferably at least 5%, which occursduring the phase transformation particularly at relatively constantstress.

[0054]FIG. 6 graphically illustrates the relationship of the finalaustenite transformation temperature (Af) of a binary NiTi alloy of theinvention with the temperature to which the alloy is subjected duringthe heat treatment. The amount of cold work and composition will shiftthe curve to the other valves but will not change the general shape ofthe curve.

[0055] Because of the extended strain range under stress-induced phasetransformation which is characteristic of the superelastic materialdescribed herein, a guidewire made at least in part of such material canbe readily advanced through tortuous arterial passageways. When theguidewire or guiding member of the invention engages the wall of a bodylumen such as a blood vessel, it will deform and in doing so willtransform the austenite of the superelastic portion to martensite. Uponthe disengagement of the guidewire or guiding member, the stress isreduced or eliminated from within the superelastic portion of theguidewire or guiding member and it recovers to its original shape, i.e.the shape “remembered” which is preferably straight. The straight“memory” in conjunction with little or no nonuniform residual stresseswithin the guidewire or guiding member prevent whipping of the guidewirewhen it is torqued from the proximal end thereof. Moreover, due to thevery high level of stress needed to transform the austenite phase to themartensite phase, there is little chance for permanent deformation ofthe guidewire or the guiding member when it is advanced through apatient's artery.

[0056] The present invention provides guiding members for guidewires andfixed wire catheters which have superelastic characteristics tofacilitate the advancing thereof in a body lumen. The guiding membersexhibit extensive, recoverable strain resulting from stress inducedphase transformation of austenite to martensite at exceptionally highstress levels which greatly minimizes the risk of damage to arteriesduring the advancement therein.

[0057] The superelastic tubular members of the present invention areparticularly attractive for use in a wide variety of intravascularcatheters, such as fixed-wire catheters wherein the Nitinol hypotubinghaving superelastic properties may be employed to direct inflation fluidto the interior of the dilatation balloon. In this case a guiding membermay be secured to the distal end of the superelastic Nitinol tubing andextend through the interior of the inflatable balloon and out the distalend thereof. The guiding member may also be made of the superelasticNitinol of the invention.

[0058] Superelastic hypotubing generally has been found to have aslightly lower stress level compared to wire when the austenite istransformed to martensite. However, this stress level is above 40 ksiand is usually above 70 ksi. The hypotubing of the invention generallymay have an outer diameter from about 0.05 inch down to about 0.006 inch(1.27-0.152 mm) with wall thicknesses of about 0.001 to about 0.004inch,. (0.0254-0.102 mm) A presently preferred superelastic hypotubinghas an outer diameter of about 0.012 inch (0.31 mm) and a wall thicknessof about 0.002 inch (0.051 mm).

[0059] While the above description of the invention is directed topresently preferred embodiments, various modifications and improvementscan be made to the invention without departing therefrom.

What is claimed is:
 1. An intravascular guidewire comprising a) anelongated member having a proximal portion and a distal portion andbeing formed at least in part of a superelastic alloy consistingessentially of about 30 to about 52% titanium, about 38 to about 52%nickel and up to 20% additional alloying elements selected from thegroup consisting of iron, cobalt, platinum, palladium, copper andvanadium, said alloy part having an austenite phase which has a finaltransformation temperature below about 45° C., which transforms to amartensite phase upon the application of stress and which has beenthermomechanically formed in a procedure which includes a final coldworking followed by a heat treatment at a temperature between about 450°to about 600° C. while applying tension to the cold worked elongatedmember; and b) torquing means on the proximal end of the elongatedmember.
 2. The guidewire of claim 1 wherein the alloy contains one ormore additional alloying elements selected from the group consisting ofiron, cobalt, platinum and palladium in amounts of up to about 3%. 3.The guidewire of claim 1 wherein the alloy contains one or moreadditional alloying elements selected from the group consisting ofcopper in amounts of up to 12% and vanadium in amounts of up to about10%.
 4. The guidewire of claim 1 wherein the temperature of the heattreatment is between about 475° and about 550° C.
 5. The guidewire ofclaim 1 wherein the superelastic portion has a straight memory at atemperature less than about 45° C.
 6. The guidewire of claim 1 whereinthe final cold worked alloy part is mechanically straightened before theheat treatment.
 7. A superelastic alloy body having an austenite phasewhich is stable at a desired operating temperature and which willtransform to martensite phase upon the application thereto of stress,exhibiting a recoverable strain of at least about 4% upon the stressinduced transformation of the austenite phase to martensite phase andhaving been formed by thermomechanical processing which includes a finalcold working about 10 to about 75% and a memory imparting heat treatmentat a temperature between about 475° and about 600° C.
 8. The body ofclaim 7 wherein the strain of the body during the stress inducedtransformation of the austenite phase to the martensite phase is withinthe range of about 2% to about 8%.
 9. The body of claim 7 wherein theaustenite-to-martensite transformation occurs at a relatively constantstress above about 50 ksi.
 10. The body of claim 7 wherein theaustenite-to-martensite transformation occurs at a relatively constantstress above about 70 ksi.
 11. The body of claim 7 wherein a distalportion thereof has a plurality of sections which have progressivelysmaller cross-sections in the distal direction.
 12. The body of claim 7further comprising a flexible member disposed about the superelasticdistal portion thereof.
 13. The body of claim 12 wherein the flexiblemember is a helical coil with a rounded plug on the distal end thereof.14. The body of claim 6 wherein tension is applied to the elongated bodywhile being subjected to the memory imparting heat treatment.
 15. Thebody of claim 6 wherein the body is subjected to mechanicalstraightening between the cold working and heat treating steps.
 16. Afixed-wire balloon angioplasty catheter comprising: a) an elongatedcatheter body with an inner lumen extending therein; b) an inflatableballoon on the distal extremity of the catheter body and having aninterior in fluid communication with the inner lumen of the catheterbody; and c) a guiding member which is disposed at least in part withinthe interior of the inflatable balloon and which is formed at least inpart of a superelastic alloy body having an austenite phase which isstable at a desired operating temperature and which will transform tomartensite phase upon the application thereto of stress, exhibiting arecoverable strain of at least about 4% upon the stress inducedtransformation of the austenite phase to martensite phase and havingbeen formed by a thermomechanical processing which includes a final coldworking of about 10 to about 75% and then a memory imparting heattreatment at a temperature between about 450° and about 600° C. whiletension is applied thereto.
 17. The fixed wire balloon angioplastycatheter of claim 17 wherein the thermomechanical processing includes amechanical straightening between the cold working and heat treating. 18.A method of forming a superelastic elongated member being in anaustenite phase which is stable at temperatures less than about 45° C.:providing an elongated member formed of an alloy consisting essentiallyof about 30 to about 52% titanium, about 38 to about 52% nickel and upto a total of about 20% of one or more additional alloying elementsselected from the group consisting of iron, cobalt, chromium, platinum,palladium, copper, vanadium, zirconium, hafnium and niobium; subjectingthe elongated member to thermomechanical processing which includes afinal cold working of about 10 to about 75% and a heat treatment at atemperature between about 450° and about 600° C. while subjecting theelongated member to a tension of up to about 50% of the room temperaturetensile strength.
 19. The method of claim 18 wherein the elongatedmember is subjected to mechanical straightening after the final coldworking but before the heat treatment.
 20. The method of claim 18wherein the heat treating temperature is between about 475° and about550° C.
 21. The method of claim 18 wherein the final cold-worked numberis heat treated for about 0.5 to about 60 minutes.
 22. An elongatedtubular body suitable for use within a human body which has acylindrical wall defining an inner lumen therein, which is formed of asuperelastic alloy consisting essentially of about 30 to about 52%titanium, about 38 to 52% nickel, and to about 20% of one or moreelements selected from the group consisting of iron, cobalt, chromium,platinum, palladium, copper, vanadium, zirconium, hafnium and niobium ina stable austenite phase which will transform to martensite phase uponthe application of stress, which will exhibit a recoverable strain of atleast about 4% from the application of stress which transforms theaustenite phase to the martensite phase and which has been fabricated bya thermomechanical processing which includes a final cold working ofabout 10 to about 75% and then a memory imparting heat treatment at atemperature of about 450° to about 600° C.
 23. The tubular body of claim22 wherein the stress level at which the austenite phase transforms tothe martensite phase is above 50 ksi.
 24. The tubular body of claim 22wherein the austenite-to-martensite transformation occurs at arelatively constant yield stress above about 70 ksi.
 25. The tubularbody of claim 22 wherein the alloy contains at least one elementselected from the group consisting of up to about 20% copper and up toabout 10% vanadium.
 26. The tubular body of claim 22 wherein the alloycontains at least one element selected from the group consisting ofiron, cobalt, palladium and platinum in amounts up to about 3%.
 27. Thetubular body of claim 24 having an outer diameter of about 0.006 toabout 0.05 inch and a wall thickness of about 0.001 to about 0.004 inch.28. A method of forming a superelastic elongated member having astraight memory comprising subjecting an elongated member having acomposition consisting of a predominant amount of NiTi intermetallicconstituent to thermomechanical processing which includes a final coldworking of about 10 to about 75% and a heat treatment at a temperaturebetween about 450° and about 600° C. while subjecting the cold workedelongated member to sufficient tension to ensure a straight memory. 29.The method of claim 28 wherein the elongated member is formed of analloy consisting essentially of about 30 to about 52% titanium, abut 38to about 52% nickel and up to a total of about 10% of one or moreadditional alloying elements selected from the group consisting of iron,cobalt, chromium, platinum, palladium, copper, vanadium, zirconium,hafnium and niobium.
 30. The method of claim 29 wherein the elongatedmember is in the austenite phase.
 31. The method of claim 29 wherein theheat treatment is at a temperature between about 475° to about 550° C.32. The method of claim 29 wherein the elongated member is mechanicallystraightened after the final cold working and before the heat treating.